A poetry written by an unknown author listed Eustace among the crusaders from the Diocese of Thérouanne. According to the same source, he was a military official in the castle of Beaurainville in the County of Saint-Pol. He was a rear-vassal of Eustace III of Boulogne, because the counts of Saint-Pol held Beaurainville in fief from the Counts of Boulogne. Eustace's surname implies that either Eustace or one of his ancestors was responsible for the management of a store-house. Historian Alan V. Murray says, Eustace most probably came to the Holy Land in the retinue of his lord, Hugh II of Saint-Pol, during the First Crusade. Jonathan Riley-Smith writes that Eustace arrived to the Kingdom of Jerusalem after 1099.
Eustace became one of the most trusted officials of the younger brother of Eustace III of Boulogne, Baldwin I of Jerusalem. His participation in the Third Battle of Ramla in August 1105 was the first recorded event of his life in the Kingdom of Jerusalem. Baldwin appointed him and Pagan of Haifa to start negotiations with Bertrand of Toulouse, William Jordan of Cerdanya and Tancred, the regent of Antioch, about the organization of a conference where they could resolve their conflicts. The crusader leaders assembled near Tripoli and reached a compromise in June 1109. Their reconciliation enabled their united armies to force the defenders of Tripoli to surrender on 26 June. Baldwin I granted Caesarea to Eustace before September 1110. Eustace also received Sidon, which was captured by the united forces of Baldwin I and Sigurd I of Norway on 5 December. He participated in Baldwin I's military campaigns against Shaizar in 1111. In the same year, he financed the building of siege machines during the unsuccessful siege of Tyre. He married Emma, the niece of Arnulf of Chocques, the Latin Patriarch of Jerusalem. Arnulf gave Jericho to Eustace as Emma's dowry, although the town had been the property of the Church of the Holy Sepulchre. The lordships of Caesarea and Sidon and his wife's dowry made Eustace the most powerful noblemen in the entire kingdom. He also had preeminent position in the royal council. He was one of the four secular lords to attend the legislative assembly that Baldwin I's successor, Baldwin II of Jerusalem, and Warmund, Patriarch of Jerusalem held at Nablus on 16 January 1120. The assembly passed decrees that regulated the collection and spending of tithes and ordered the persecution of adultery, procuring, homosexuality, bigamy and sexual relations between Christians and Muslims.
Regent
The Artuqid prince Belek Ghazi captured Baldwin II on 18 April 1123. Patriarch Warmund convoked an assembly to Acre where Eustace was elected constable and bailiff to administer the kingdom during the king's captivity. Baldwin took possession of the fortress of Kharpurt where he had been held in captivity. Eustace soon sent reinforcements to Kharpurt to assist the king, but Balak ibn Bahram recaptured the fortress by the time the crusader troops reached it. A Fatimid army invaded the kingdom from Ascalon in May 1123. Eustace attacked the invaders near Ibelin and defeated them on 29 May. He did not long survive his victory, because he died on 15 June 1123. He was buried in the Church of Saint Mary of the Latins. According to Fulcher of Chartres he was "a stout man of honest character," and "a wise and prudent man, with great experience in military matters" according to William of Tyre.
Family
With Emma, Eustace was the father of the twins, Gerard and Walter, who succeeded him in Sidon and Caesarea respectively. Emma married Hugh II of Le Puiset, Count of Jaffa. The relationship between her second husband and her sons was tense.
